1. Field of the Invention
This invention relates to an improved process for producing liquid hydrocarbons from coal. More particularly, this invention relates to a process for enhancing liquid hydrocarbon yields from solid coal by subjecting the coal to suitable liquefaction conditions, recovering a bottoms stream comprising unconverted coal which is then subjected to alkylation or acylation and subjecting the alkylated/acylated bottoms to further liquefaction.
2. Discussion of the Prior Art
In recent years, the production of liquid hydrocarbons from non-petroleum sources has taken on added importance. Thus, with proven world petroleum reserves shrinking, other forms of energy have attracted attention. Perhaps, the greatest attention has been directed to coal, an abundant fossil fuel, particularly in the United States, which can be converted to liquid hydrocarbons at costs approaching current and projected costs for the refining of crude petroleum. Moreover, basic coal conversion technology exists and has been demonstrated on a variety of levels, e.g., pilot plant and full scale commercial (although highly expensive) plants. However, full development of existing conversion technology is only now underway.
Coking of coal with the attendant recovery of coal liquids is a long established process. Solvation of coal, with or without the addition of molecular hydrogen has also long been known as a feasible, if not economically attractive, process for producing coal liquids (see, for example, U.S. Pat. No. 1,342,790). The Pott-Broche Process (for example, U.S. Pat. No. 1,881,977) with modifications, was capable of producing gasoline from coal, albeit at then excessive costs. A number of process schemes for the liquefaction of coal using hydrogen donor solvents has also been suggested (for example, U.S. Pat. No. 3,617,513).
While there has been great emphasis on the conversion of coal to more useful liquid and gaseous products the investigation of the coal molecule, i.e., that which is to be converted, has often lagged and has been of relatively little importance. Nevertheless, an understanding of the material to be converted is elementary to the development of sound conversion technology. As a result, the chemistry of coal is now being actively pursued and while the structure of coal remains, for the most part, unresolved, it is now generally believed that the coal molecule is not constructed on a diamond-like framework but rather it contains aromatic rings which are highly substituted (i.e., fused to other aromatics or hydroaromatics, or attached to alkyl, ether, hydroxyl, etc. groups). Additionally, it is now believed that coal exhibits secondary structural characteristics such as hydrogen bonding, interaromatic ring bonds, etc., which generate the three-dimensional structure of coal. As a result of the condensed ring structure of coal, liquefaction processes have generally been limited by their ability to solvate exposed areas of the coal molecule. Thus, under normal liquefaction conditions, the secondary structural characteristics of the coal molecule are only partially, if at all, destroyed and a significant portion of the coal is not converted in the liquefaction process.
In copending application Ser. No. 635,706 filed Nov. 26, 1975 it has been disclosed that pretreatment of coal by alkylation or acylation can affect the secondary structural characteristics of coal and provide additional reaction sites for the liquefaction reaction. The copending application also discloses that the bottoms fraction from the liquefaction reaction can be further alkylated/acylated and then again subjected to liquefaction conditions. It has now been found that increased liquid hydrocarbon yields can be obtained from solid coal by alkylating/acylating the recovered bottoms streams from a liquefaction reaction zone. Thus, enhanced liquid yields can be obtained by alkylating/acylating a much smaller coal-containing stream than was previously thought possible. In the copending application, the entire coal feed was subjected to alkylation/acylation while this invention teaches that only the bottoms stream need be treated to obtain increased liquid yields.
It is believed that the highly refractory nature of the coal bottoms can be broken down by the introduction of alkyl or acyl radicals into the coal molecule. Thus, additional reactive sites are created in the converted coal which are susceptible to conversion during liquefaction.
Coal has been alkylated primarily for investigation of the coal molecule. See, for example, C. Kroger, Forshungs Ber. Nordrhein-Westfalen No. 1488 (1965); H. W. Sternberg and C. L. Delle Donne, Fuel, 53, 172 (1974); H. W. Sternberg, C. L. Delle Donne, P. Pantages, E. C. Moroni and R. E. Markby, Fuel, 50, 432 (1971); J. D. Spencer and B. Linville, Bureau of Mines Energy Program, 1971, Bureau of Mines 1C8551, 1972; B. Linville and J. D. Spencer, Review of Bureau of Mines Energy Program, 1970, Bureau of Mines 1C8526, 1971; W. Hodek and G. Kolling, Fuel, 52, 220 (1973) discuss the increase in extractability of bituminous coal by the related Friedel-Crafts acylation. Nevertheless, no prior reference has suggested that increased yields of liquid products via liquefaction can be obtained by first subjecting the coal to either alkylation or acylation. See, also, F. Meyer, Ph.D. Thesis, University of Munster, 1969; J. D. Spencer, Review of Bureau of Mines Coal Program, 1968, Bureau of Mines, 1C8416, 1969; J. D. Spencer, Review of Bureau of Mines Coal Program, 1969, Bureau of Mines, 1C8385, 1968, Sternberg, H. W. et al, The Electrochemical Reduction of a Low Volatile Bituminous Material, Fuel, 45 (6) 409-482 (1966). In "Coal Liquefaction by Alkylation Techniques," D. D. Denson and D. W. Buckhouse in a Stanford Research Institute paper dated June 20, 1975 prepared under a National Science Foundation grant, alkylation was utilized to enhance solvent refining but, again, no mention was made of enhancing liquid product yields by converting the coal under liquefaction conditions.